Zinc-based plated steel sheet such as galvannealed steel sheet and hot-dip galvanized steel sheet has excellent corrosion resistance. For this reason, zinc-based plated steel sheet is used in a wide range of applications including steel sheet for automobile bodies, steel sheet for household appliances, steel sheet for industrial machines, steel sheet for furniture, and steel sheet for building materials. Zinc-based plated steel sheet is typically formed into a product such as an automobile body by the following steps (a)-(d).
(a) The plated steel sheet is formed into a prescribed shape by subjecting to press forming, for example,
(b) The outer periphery of the formed member is superimposed on a second member to which it is to be welded, and the two members are secured to each other in this state.
(c) The two members are welded by resistance welding such as spot welding or seam welding, TIG welding, MIG welding, or plasma arc welding in which the portion where the first member is superimposed on the second member becomes a weld zone. As is well known, particularly in the case of an automobile body, spot welding is frequently used as a welding method.
(d) Welding is repeatedly carried out in this manner.
Laser welding using a laser beam has attracted attention as a welding technique for zinc-based plated steel sheet. FIG. 39(a) FIG. 39(b) are explanatory views showing the manufacture of a laminated steel sheet 2 formed from zinc-based plated steel sheets 1. As shown in FIG. 39(a), two zinc-based plated steel sheets 1, 1 are superimposed and secured to each other. Then, as shown in FIG. 39(b), laser welding is performed on the two zinc-based plated steel sheets 1, 1 to form a weld 2a, thereby manufacturing a laminated steel sheet 2.
Each zinc-based plated steel sheet 1 has a plated coating formed on the surface thereof, the plated coating predominantly comprising zinc, which has a boiling point which is lower than the melting point of steel. Therefore, when laser welding is carried out, the zinc, which is the main component of the plated coating formed on the superimposed surfaces, is explosively vaporized by the input of heat during laser welding. Accordingly, when two zinc-based plated steel sheets 1 are simply superimposed and laser welding is carried out as shown in FIGS. 39(a) and 39(b), or when a zinc-based plated steel sheet 1 and an ordinary steel sheet are simply superimposed and laser welding is carried out, molten iron which is formed by the input of heat during laser welding is blown away by the zinc which explosively vaporizes, and weld defects are formed in the weld 2a. 
In order to prevent the occurrence of such weld defects and obtain a good weld, inventions are known in which a gap having suitable dimensions through which zinc which is vaporized by the input of heat during laser welding can be discharged to the outside is provided in a location where a laser welding is to be performed on two superimposed steel sheets, at least one of which is a zinc-based plated steel sheet.
For example, in JP-A 10-216974, Japanese Patent No. 2,571,976, and in the publication: Nikkei Mechanical, Sep. 30, 1996, No. 490 and the like, inventions are disclosed in which projections are previously formed by press forming, for example, on a zinc-based plated steel sheet which is a material for laser welding, and laser welding is carried out while maintaining a gap of suitable dimensions between two superimposed steel sheets by means of the projections.
However, the suitable dimensions of this gap are extremely small, such as on the order of 0.05-0.3 mm for a zinc-based plated steel sheet with a thickness of 0.7 mm. Therefore, when carrying out the above-described prior art inventions, it is necessary to form these minute projections on either of the two steel sheets by press forming. For this purpose, it becomes necessary to carry out precision machining or hand finishing of the surface of the pressing dies elaborately. These operations become extremely difficult and entail large costs. In particular, there are many cases in which the surface to be welded of parts of automobile bodies made from zinc-based plated steel sheets are given a complicated, curved three-dimensional shape, so carrying out precision machining or hand finishing of a pressing die elaborately is actually impossible.
In addition, the outline of the ridgeline of the minute projections which are formed by press forming can easily become indistinct due to elastic recovery, which inevitably occurs in a formed article after press forming. Therefore, even if it is possible to carry out machining of a pressing die so as to provide desired minute projections, it is not easy to provide a press formed article with projections having the same shape as the projections of the die.
Furthermore, projections which are provided in a pressing die can easily change shape due to abrasion accompanying press working. Therefore, it is difficult to stably maintain a gap having suitable dimensions for laser welding over a long period.
In JP-A 2001-162388, an invention is disclosed in which projections which are overly tall with respect to the appropriate dimensions of a gap are previously formed on a steel sheet, and laser welding is carried out in the location of a suitable gap in a sloping region formed between the projections and the rod of a clamping apparatus. In that invention, the height of the projections is set to be larger than the actual target value, so the projections can be stably provided by machining.
In that invention, laser welding is carried out in the sloping region formed between the projections, which are set to have a large height, and a clamping rod. Therefore, if the position where laser welding is carried out even slightly moves in the horizontal direction, the size of the gap greatly varies. Accordingly, although projections can be stably formed in that invention, it is not possible to always keep the dimensions of a gap for laser welding sufficient to stably guarantee the quality of laser welding.